For charged particle beams such as electron beam systems and focused ion beam (FIB) systems, beam quality is very important to imaging performance. In particular the uniformity of particle kinetic energy is an important quality. Ideally, all particles in the beam have the exact same kinetic energy. Particles having different energies focus at different points, thereby enlarging the spot size of the beam on the work piece and reducing resolution. This is referred to as chromatic aberration. In order to evaluate the uniformity of the energies of the particles in the beam, it is important to be able to measure the distribution of particle kinetic energy in the beam. It can also be important to determine the absolute energy of the particles in the beam. The absolute energy and the energy distribution are typically measured by a retarding field analyzer (RFA).
To measure the beam energy and/or energy distribution, the RFA is temporarily inserted into the charged particle beam system downstream of the focusing column. The retarding field strength is increased incrementally, and the beam current that passes through the retarding field after each incremental field strength increase is measured. Eventually, the incremented retarding field will be sufficiently strong to stop all particles in the beam from passing through the field. RFAs typically include a filter lens or a grid to which a voltage is applied to produce the retarding field. Using an RFA requires mounting additional equipment after the focusing column, which entails additional costs. The RFA requires additional vacuum electrical feed-throughs and wiring to connect the additional equipment, extra power supplies that are highly regulated, and some type of beam detector. Significant time is required to place the RFA into operation.
Because the beam energy at the retarding element is very low, the beam is easily disturbed, making precise beam alignment important to reduce measurement error. RFAs typically direct the beam into a Faraday cup and do not have imaging capability, so it can be difficult to verify that the beam is accurately aligned with the RFA.